--- title: JavaScript Const description: "## ECMAScript 2015 ES2015 introduced two important new JavaScript keywords: `let` and `const`. Variables defined with `const` behave like `let` variables, except they cannot be reassigned: ### Example `const PI = 3.141592653589793; PI = 3.14; // This will give an error PI = PI + 10; //…" url: "https://apprentice.mn/docs/javascript-const/" updated: "2021-05-12" category: Javascript --- # JavaScript Const ## ECMAScript 2015 ES2015 introduced two important new JavaScript keywords: `let` and `const`. Variables defined with `const` behave like `let` variables, except they cannot be reassigned: ### Example `const PI = 3.141592653589793; PI = 3.14; // This will give an error PI = PI + 10; // This will also give an error` ## Block Scope Declaring a variable with `const` is similar to `let` when it comes to **Block Scope**. The x declared in the block, in this example, is not the same as the x declared outside the block: ### Example `var x = 10; // Here x is 10 { const x = 2; // Here x is 2 } // Here x is 10` You can learn more about [Block Scope](https://www.w3schools.com/js/js_let.asp) in the previous chapter: [JavaScript Let](https://www.w3schools.com/js/js_let.asp). --- ## Assigned when Declared JavaScript `const` variables must be assigned a value when they are declared: ### Incorrect `const PI; PI = 3.14159265359;` ### Correct `const PI = 3.14159265359;` > ## Not Real Constants > > > > The keyword `const` is a little misleading. > > > > It does NOT define a constant value. It defines a constant reference to a value. > > > > Because of this, we cannot change constant primitive values, but we can change the properties of constant objects. --- ## Primitive Values If we assign a primitive value to a constant, we cannot change the primitive value:  ### Example `const PI = 3.141592653589793; PI = 3.14; // This will give an error PI = PI + 10; // This will also give an error` ## Constant Objects can Change You can change the properties of a constant object: ### Example `// You can create a const object: const car = {type:"Fiat", model:"500", color:"white"}; // You can change a property: car.color = "red"; // You can add a property: car.owner = "Johnson";` But you can NOT reassign a constant object: ### Example `const car = {type:"Fiat", model:"500", color:"white"}; car = {type:"Volvo", model:"EX60", color:"red"}; // ERROR` ## Constant Arrays can Change You can change the elements of a constant array: ### Example `// You can create a constant array: const cars = ["Saab", "Volvo", "BMW"]; // You can change an element: cars[0] = "Toyota"; // You can add an element: cars.push("Audi");` But you can NOT reassign a constant array: ### Example `const cars = ["Saab", "Volvo", "BMW"]; cars = ["Toyota", "Volvo", "Audi"]; // ERROR` ## Browser Support The `const` keyword is not supported in Internet Explorer 10 or earlier. The following table defines the first browser versions with full support for the `const` keyword: | ![](https://www.w3schools.com/images/compatible_chrome.gif) | ![](https://www.w3schools.com/images/compatible_edge.gif) | ![](https://www.w3schools.com/images/compatible_firefox.gif) | ![](https://www.w3schools.com/images/compatible_safari.gif) | ![](https://www.w3schools.com/images/compatible_opera.gif) | | --- | --- | --- | --- | --- | | Chrome 49 | IE / Edge 11 | Firefox 36 | Safari 10 | Opera 36 | | Mar, 2016 | Oct, 2013 | Feb, 2015 | Sep, 2016 | Mar, 2016 | ---     --- ## Redeclaring Redeclaring a JavaScript `var` variable is allowed anywhere in a program: ### Example `var x = 2; // Allowed var x = 3; // Allowed x = 4; // Allowed` Redeclaring or reassigning an existing `var` or `let` variable to `const`, in the same scope, or in the same block, is not allowed: ### Example `var x = 2; // Allowed const x = 2; // Not allowed { let x = 2; // Allowed const x = 2; // Not allowed }` Redeclaring or reassigning an existing `const` variable, in the same scope, or in the same block, is not allowed: ### Example `const x = 2; // Allowed const x = 3; // Not allowed x = 3; // Not allowed var x = 3; // Not allowed let x = 3; // Not allowed { const x = 2; // Allowed const x = 3; // Not allowed x = 3; // Not allowed var x = 3; // Not allowed let x = 3; // Not allowed }` Redeclaring a variable with `const`, in another scope, or in another block, is allowed: ### Example `const x = 2; // Allowed { const x = 3; // Allowed } { const x = 4; // Allowed }` ## Hoisting Variables defined with `var` are **hoisted** to the top and can be initialized at any time (if you don’t know what Hoisting is, read our [Hoisting Chapter](https://www.w3schools.com/js/js_hoisting.asp)). Meaning: You can use the variable before it is declared: ### Example This is OK: `carName = "Volvo"; alert(carName); var carName;` Variables defined with `const` are hoisted to the top of the block, but not initialized. Meaning: The block of code is aware of the variable, but it cannot be used until it has been declared. The variable is in a “temporal dead zone” from the start of the block until it is declared. Using a `const` variable before it is declared, is a syntax errror, so the code will simply not run. ### Example This code will not run: `carName = "Volvo"; const carName;` [https://www.addtoany.com/add_to/facebook?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fjavascript-const%2F&linkname=JavaScript%20Const](https://www.addtoany.com/add_to/facebook?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fjavascript-const%2F&linkname=JavaScript%20Const)[https://www.addtoany.com/add_to/mastodon?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fjavascript-const%2F&linkname=JavaScript%20Const](https://www.addtoany.com/add_to/mastodon?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fjavascript-const%2F&linkname=JavaScript%20Const)[https://www.addtoany.com/add_to/email?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fjavascript-const%2F&linkname=JavaScript%20Const](https://www.addtoany.com/add_to/email?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fjavascript-const%2F&linkname=JavaScript%20Const)[https://www.addtoany.com/share](https://www.addtoany.com/share)